crypto: rockchip - fix scatterlist nents error

In some cases, the nents of src scatterlist is different from
dst scatterlist. So two variables are used to handle the nents
of src&dst scatterlist.

Reported-by: Eric Biggers <ebiggers@google.com>
Fixes: 433cd2c6 ("crypto: rockchip - add crypto driver for rk3288")
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Zhang Zhijie <zhangzj@rock-chips.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>